1. Field of the Invention
The invention relates to a new concept for the laying of transmission, conduction and communication lines, and particularly, it details improvements in the laying of channeling lines for applications such as signal transmission, energy and fluid conduction and other purposes.
The invention has been particularly developed for its application in the laying of optical fibers within a channeling line placed on towers for high voltage transmission lines. In application, however, the new channeling unit may be applied to other transmission, conduction and communication applications which may be continued within the cavity of a specially designed tubular channeling structure. In order to simplify the description herein, the specification will be directed to the laying of optical fibers. It is to be understood, however, that the same concepts or similar concepts may be applied to other types of signal or power transmission lines and the conduction of fluids generally.
From the analysis of electric and magnetic field configurations in high voltage transmission lines, it has been discovered that there are two particular positions and a zone in which the channeling line may be most optimally placed so as to minimize the electromagnetic effects. Thus, a zone is defined that presents several advantages for the laying of channeling lines of application means. In addition, the invention further contemplates that the channeling lines may also be formed from a dielectric sheathing, and placed within the zone while maintaining the several advantages for the installation of the channeling lines.
2. Description of Related Art
The transmission over large distances of communication signals or electrical power and related applications, may be carried out by the installation or "laying" of cables. The laying is accomplished using any of the three known procedures: underground, underwater and aerial installation.
Underground installation consists of burying the cables lodged in pipes and hermetic sheaths. This method has several inconveniences, such as the high installation and operating costs particularly when it is necessary to make repairs or modifications in the network forcing the continuous excavation and covering of ditches. At the same time, underground installation may become dangerous, generally because the cables are not visible and might be cut or damaged during excavations.
Underwater installation is accomplished through the laying of submarine cables or of cables which lie in the bed of rivers and lakes. It is yet a more expensive technology than the underground method discussed above, as it may require laying cables at very deep depths in order to avoid accidents caused by large ships or other aquatic transportation systems. Further, the cables must be provided with hermetic sheaths of great resistance to the hydraulic pressures and to the degradation produced by the underwater environment.
Aerial installation is accomplished through the use of poles, towers or other supports which hold in place the cables and which are arranged at a certain distance from the ground level, designated as the "safety level." The access to an aerial system is easier and quicker than underground or underwater cables, making possible the direct repairs and modifications of the network and of its connections. The laying of optical fibers on high voltage overhead lines would allow the set up of a communications network of high quality, using the towers of preexistent electrical networks and their right-of-way, with a noticeably lower cost than the cost of an underground installation.
The aerial laying of optical fibers may be carried out utilizing the following techniques:
Laying Over High Voltage Conductors
The tower suspended, high voltage power line systems generally include at the upper part of the tower at least one metallic cable designated as being a security cable or "guard wire". The aim of this security cable is to protect the high voltage network and its supporting structures from possible atmosphere electric discharges such as lighting. The determination of the characteristics of such a cable are governed by two main conditions:
Structural requirements, taking into account the span length, or the distance between towers, and the action of the wind, ice and snow loads. PA1 Electric requirements, taking into account the short-circuit current and the potential maximum current pulse of a lightning strike. PA1 (a) Degradation of the dielectric sheath of the cable by electromagnetic effects which has forced the use of the method only on lines of the same voltages or lower than 66 KV, or in aerial lines with middle and high tension superior to 50 KV. PA1 (b) Degradation of the external sheath due to the environmental conditions, the action of the wind, erosion, solar radiation, rain and ice, etc. PA1 (c) Degradation of the external sheath due to the effect of the regional fauna such as mainly, the birds which sit on or peck the sheath.
For the laying of optical fibers, one of the known methods contemplates replacing the guard wire with a specially designed combination of a security cable and optical fibers. In this apparatus, the security cable performs the function of the guard wire and supports the optical fibers, simultaneously.
At present, with respect to the installation method for such a combination cable/optical fiber assembly, there are three options. First, installation on un-energized lines may be accomplished when a new power line system is installed, and the cable/optical fiber is incorporated during the procedure of the high voltage line installation.
Second, for the installation to be carried out on an operational preexistent power line system, to replace the guard wire with a combination cable/optical fiber, it is necessary to disconnect the electric supply to the power lines to carry out the installation. This situation involves an economic loss due to the stoppage of the electric service, and to the corresponding payments for such service.
Third, in order to avoid inconveniences caused by the interruptions in the services, it is possible to accomplish installation with the lines energized on an operational preexistent power line system. However, special installation techniques must be used which allow the laying of combination guard wire cable/optical fibers with the high voltage lines energized. These methods are more expensive and slower, and involve higher risks to workers installing the cables.
Laying below the high voltage conductors may be possible using a cable made up of dielectric materials, which protect and support the optical fibers when they are laid below the energized conductors. This method makes possible installation with the energized lines at a lower cost, with less interruptions in the service and with a lower load on the towers by the effects of the wind due to the lower height at which the cable is held.
In order to support the cable, there exist two options. First, a dielectric cable may be self-supported using a central core of aramidic fiber such as the product sold by the DuPont corporation under the tradename "KEVLAR," to support the mechanical stress. Support may also be provided by a "messenger cable" of aramidic fiber which supports the dielectric cable which contains the optical fibers.
In either of these options, the cable design must take into account the structural requirements, the function of the length of the span (or distance between the towers), and the action of the loads caused by atmospheric factors (wind, ice and snow). Nevertheless, the use of dielectric materials on the sheath of the optical cable makes it necessary to take into account, particularly, the environmental electromagnetic conditions, which generate the following inconveniences:
On the other hand, the existence of structural limitations due to the cable design, or its use in spans superior to 100 m, has not been allowed in layings with spans superior to 150 m.
The installation of metallic channeling lines in an aerial, high voltage, transmission system causes changes in the impedance of the line under normal operating conditions. These changes are due to the capacitive (electric) and inductive (magnetic) coupling between the power carrying conductors of the high voltage transmission system and the channeling line. Moreover, the inductive coupling causes parasitic currents to flow in or on the channeling line. The effects of these couplings are not cancelled by the transposition of the power line conductors, since, for safety reasons, the channeling line has to be grounded at each support tower. On the other hand, dielectric covered channeling lines present several inconveniences. In particular, the degradation of the dielectric cover due to the effects of the electric field surrounding the power conductors.